Selective laser sintering (“SLS”) is an additive manufacturing technique that uses electromagnetic radiation from a laser to fuse small particles of plastic, metal (direct metal laser sintering), ceramic, or glass powders into a mass that has a desired three dimensional object. The laser selectively fuses powdered material by scanning cross-sections generated from a 3-D digital description of the part on the surface of a powder bed. After a cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied, and the bed is rescanned. This process is repeated until the part is completed.
An SLS machine typically preheats the material in the powder bed to just below the melting point of the powder. This is typically accomplished by preheating the actual bed, which then transfers energy to the powder. The preheating of the powder makes it easier for the laser to raise the temperature of the selected regions of layer of unfused powder to the melting point. When working with polymer powders in the SLS process, the bed temperature is set to a temperature specific to the polymer resin in use. This specified temperature is typically close to the melting point of the resin. The laser causes fusion of the powder only in locations specified by the input. Laser energy exposure is typically selected based on the polymer in use and is between the amount required to melt the resin and the amount that will cause degradation. Preheating of the material inhibits unwanted distortions in formed parts as they cool.
After the layer-wise process is completed, the formed object(s) is in a cake of unfused powder, referred to as the cake. The formed object is extracted from the cake. The powder from the cake is recovered, sieved, and combined with unused powder and used in a subsequent SLS process. The extent that used polymer can be reused in a subsequent SLS processes is typically determined by the nature of the polymer itself. The ratio of recycled powder to unused powder is typically referred to as a recycle rate. The recycle rate of a specific polymer is typically determined by the vendor of that polymer. If the recycle rate is exceeded, the material properties of the built part can be degraded. The purpose of recycling powder in this way is to improve process economics. For example, in the case of Polyamide 11 (PA11) and other similar polymers, the recycle rate is typically below 33%. In other words, the ratio of used powder to unused powder cannot exceed 1 part recycled polymer to 2 parts unused polymer. Typically, the use of recycled powder is generally expected to degrade the material properties of parts sintered therefrom as compared to parts sintered entirely from an unused powder.
Polyaryletherketones (“PAEK”) are of interest in the SLS process because parts that have been manufactured from PAEK powder or PAEK granulates are characterized by a low flammability, a good biocompatibility, and a high resistance against hydrolysis and radiation. The thermal resistance at elevated temperatures as well as the chemical resistance distinguishes PAEK powders from ordinary plastic powders. A PAEK polymer powder may be a powder from the group consisting of polyetheretherketone (“PEEK”), polyetherketone ketone (“PEKK”), polyetherketone (“PEK”), polyetheretherketoneketone (“PEEKK”) or polyetherketoneetherketoneketone (“PEKEKK”).
A disadvantage of parts manufactured from PAEK using the SLS process is that the strength a part comprising sintered PAEK is less than the strength of the same part comprising extruded PAEK or molded PAEK. It is known that the weakest aspect of objects made from the SLS process is the mechanical performance in the out-of-plane direction. This is also called the interlaminar or Z-axis. This collectively refers to the anisotropic nature of parts made by the SLS process, with the adhesion of the layers forming the part being the weakest link. The Z-axis performance can be markedly lower than the in plane (or XY).
Another disadvantage of PAEK materials is that they cannot be recycled for use in the SLS process. The only commercial supplier of a branded SLS PAEK powder has published that the recycle rate of its PAEK powders as 0%. It is known that use of recycled PAEK powder will cause degradation in the part.